During oil recovery operations, several wells are drilled in a reservoir. They are divided into injection wells and production wells. In most cases, water is forced into the reservoir through the injection wells to maintain the pressure difference with the surface and ensure the flow of oil through the formation; oil is pushed and replaced by the injected water. Often however, water mobility is higher than oil mobility leading to unstable fronts and viscous fingering. As a consequence, the macroscopic sweep efficiency is poor, large areas of the oil-bearing formation are not swept, and oil is left in place. Increasing the viscosity of the injected water enables to decrease the mobility ratio between water and oil and to improve macroscopic sweep efficiency. As a general rule, acrylamide based copolymers such as Hydrolyzed PolyacrylAMides (HPAM) are used because they represent the best tradeoff between viscosifying power, cost, and industrial availability. Very high molecular weight (Mw) polymers are used in order to maximize their viscosifying power.
A typical water injection system comprises one water injection pump with an injection line that injects water into wells of different hydraulic resistance. To avoid excessive flow into the wells with the lowest hydraulic resistance, a choking valve is installed at the wellhead to control the flow rate injected in each well. A choke coil, a globe valve or other throttling valve device is used for the purpose. Types of polymers used, such as acrylamide based polymers, feasible concentrations and technical solutions for mixing with injection water, for specific reservoir conditions, are in this context assumed to be known in general for the skilled person.
Description of relevant technology can be found in patent publications US 2005/252559 A1, WO 2014/178723 A1, U.S. Pat. No. 4,044,991 A, US 2003/0188787, WO 2015/092334 A2, US 2005/199298 A1, U.S. Pat. Nos. 3,514,074 A, 4,276,904, 3,170,483 A and 4,782,847.
A demand exists for improved technology for injection of water containing viscosity-enhancing polymers into reservoirs.